The present invention relates to improvements in plastic articles and methods and apparatus for extruding such articles. More particularly, the present invention relates to an extruding apparatus and extrusion method for making a novel composite extruded article formed from a plurality of compositions and having a strippable or separable portion.
The present invention relates most particularly to the field of extruded articles including the apparatus and process for making them in which a stable article is produced which will not inadvertently be separated, but which posseses an intermediate portion forming a weakened tear plane permitting the top and bottom portions of the extruded article to be separated by tearing. Articles of these physical characteristics have application to a number of commercial embodiments, but for brevity this disclosure will describe one important use of such an extruded article.
Joint formers are well known to be useful in the construction of elongated concrete masses. The primary purpose of a joint former is to form what is known as a plane of weakness in the concrete upon setting which permits or causes the concrete to crack or fracture in a controlled manner along the plane of the joint former. The joint former also may create or permit the formation of a recess in the concrete which will receive caulking material for the formation of a seal to prevent surface waters from entering the joint. Kohler U.S. Pat. No. 3,330,187 describes in some detail various joint formers and their use.
Joint formers of the type disclosed in the patent have been found to create physical stability problems which often times render such a joint former inoperative for its proposed purpose. The stability of the product produced depends upon the contact between the two separate extrusions that are brought into contact with each other while in the hot molten plastic state to permit a limited weld before the joined extrusions enter the cooling medium. Since the tensile strength of the material, is in the order of 2000 psi, it is evident that the distance between the point of contact of the two hot extrusions, the entry into the cooling medium, and the pressure with which they are brought together are very critical. If the weld is too good, or covers too much area, it is evident that it would be very difficult to separate the sections after the product has been installed and the concrete has hardened. On the other hand, if the weld is inadequate, the two elements may be so unstable as to become separated during shipment, storage, or installation.
The limited area of contact between the two elements causes the upper section to rock or tilt as it is being installed. This instability can make proper installation more difficult. The actual nature of the extrusion process contributes to the tendency of the upper section of the joint former to rock or tilt with respect to the bottom section. The plastic material as it emerges from the two orifices in the die has a tendency to "swell", due to plastic memory, with the result that the two surfaces, as they are brought together for the weld, are slightly convex with respect to each other which results in a "line" weld in much the same manner as two pieces of pipe, brought together side by side, would only establish line contact.
Alternate constructions are known wheren two thin webs would connect the upper and lower sections to enclose an empty space. From experience it is known, that as a practical matter, these webs must be on the order of 3/64 inch thick. If the webs were much thinner, the connection between the upper and lower sections would not be stable with respect to each other. Since the material which comprises the embedded lower section of the former must be of high quality, it will have a tear strength in the order of 325 lbs. or more per inch of width (or thickness). Two webs, 3/64 inch thick, would require a pulling force in the order of 30 or more lbs. in order to tear away the upper section. This task is made even more fatiguing for the person performing it by the fact that the polyvinyl chloride material comprising the webs would have an elongation before rupture in the order of 300 to 400percent. Thus, the task would be somewhat analogous to tearing a sheet of rubber.
Such an alternate design of the joint former would not allow the user the option of leaving the upper section intact in the concrete with the top surface of the upper section flush with the top surface of the concrete. Also, when subjected to such action as would be produced by the skidding or spinning action of the tires of vehicles using the paved surface, the thin connecting webs of this design will stretch allowing the upper section to rise in its recessed groove and eventually be torn away.